Underwater images play a vital role in marine engineering, marine research etc. These images can be used for control of underwater vehicles, study of corals, fishes and other specious. Underwater images suffer poor visibility and some important details may loss because of scattering and absorption. Obtaining a clear underwater image is very important and it is difficult too. Underwater image enhancement has been an active research topic over many years and several techniques has been proposed. This paper introduces a study on various techniques used to enhance the underwater images and classified the techniques into four categories.